Studies have shown, no matter how much food plot or agriculture you have, more than 50% of a deer's diet in summer will be weeds. Several species of weed are nearly as nutritious as soybeans.
And as for the effectiveness of food plots, we did a couple of studies with large tracts of unimproved land (mainly hardwoods and pine plantations) and found that it took 1% of the property converted to high-quality food plots before we saw a measurable biological response. But it took 10% converted to agriculture/food plots before improvements were dramatic (major improvements on body and antler size). Of course, so much depends on local situations. For example, what if most of the bucks you will have during hunting season don't live on your property in summer? Then food plots are doing nothing to improve those bucks. Although, even in that situation, food plots would definitely improve any local does and fawns.